1. Field
This invention relates generally to an all-fiber laser amplifier that provides high power and, more particularly, to a fiber laser amplifier including an all-fiber architecture that allows delivery of coherently combined laser power through an all-fiber beam combiner at the multi-kW level without polarization instability occurring as a result of non-linear cross-phase modulation (XPM).
2. Discussion
High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these and other applications. One known type of laser amplifier is a fiber laser amplifier that employs a doped fiber that receives a seed beam and a pump beam to amplify the seed beam and generate the laser beam, where the fiber typically has an active core diameter of about 10-20 μm.
Improvements in fiber laser amplifier designs have increased the output power of the fiber to approach its practical power and beam quality limit. To further increase the output power of a fiber amplifier some fiber laser systems employ multiple fiber laser amplifiers that combine the amplified beams in some fashion to generate higher powers. A design challenge for fiber laser amplifier systems of this type is to combine the beams from a plurality of fiber amplifiers in a manner so that the beams provide a single beam output having a uniform phase over the beam diameter such that the beam can be focused to a small focal spot. Focusing the combined beam to a small spot at a long distance (far-field) defines the quality of the beam.
In one known multiple fiber amplifier design, a master oscillator (MO) generates a seed beam that is split into a plurality of fiber seed beams each having a common wavelength, where each fiber beam is amplified. The amplified fiber seed beams are then collimated and directed to a diffractive optical element (DOE) that combines the coherent fiber beams into a single output beam. The DOE has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different angular direction are redirected by the periodic structure all of the beams diffract from the DOE in the same direction. Each fiber beam is provided to a phase modulator that controls the phase of the beam so that the phase of all the fiber beams is maintained coherent.
One known method for generating high power, near diffraction-limited beams for directed energy lasers is to utilize spectral or coherent beam combining of multiple narrow-line width fiber amplifiers. Typically, the size, weight and misalignment sensitivity of the beam combining optics scale directly with the number of fibers. Hence, maximizing power per fiber enables scaling to higher system powers in smaller and more robust packages. However, it is difficult to scale individual fiber laser amplifier powers above a few kilowatts because of numerous physical and engineering limitations, among them including stimulated Brillouion scattering (SBS), self phase modulation (SPM), spatial mode instabilities, thermal limits on pump power handling and diode pump brightness.
U.S. Pat. No. 8,488,235 issued Jul. 16, 2013 to Rothenberg, assigned to the assignee of this application and herein incorporated by reference, discloses a method for scaling per-fiber power by coherently combining the outputs of several fiber amplifiers into a single delivery fiber, which can then be fed to free-space optical beam combiners based on either SBC or CBC. In this approach, a tapered fiber bundle (TFB) fiber splitter is employed in reverse as a fiber combiner, where each input fiber to the TFB combiner is spliced to the output of a fiber amplifier. If the amplifier outputs are mutually coherent and properly phase-locked and polarization-locked using servo-control techniques, then near-100% of the input light can be combined into any one of the optic delivery fibers from the TFB combiner. The TFB combiner beam output can be directed through coherent switching to any of the output delivery fibers by changing the piston phases between the inputs. Multiple TFB combiners can be arranged to enable a single laser system to feed multiple SBC or CBC beam turrets.
A problem has been recognized with the architecture disclosed in the '235 patent that includes non-linear interactions between beams in the output delivery fiber downstream of the TFB combiner. Cross-phase modulation (XPM) between the input beams having different polarizations once the beams are co-propagating in the output delivery fiber leads to non-linear birefringence, which is a well known effect that is the basis for the Kerr shutter. In other words, as the combined beams co-propagate along the delivery fiber from the TFB combiner, they interact with each other through accumulated non-linear phase interaction causing polarization cross-talk between the beams even though they are in phase at the output of the TFB combiner. For example, the current synchronous polarization control approach employed to make the separate fiber beams co-polarized includes applying a polarization dither to each of the fiber beams prior to the TFB combiner, where that polarization dither is detected in the combined beam after the TFB combiner, and where the detected polarization is then used to adjust the polarization of the fiber beams so all the fiber beams are co-polarized. The polarization controller operates to slightly rotate or dither the state of polarization (SOP) of the particular beam back and forth so that when the beams are co-polarized, the corresponding output power will be maximized, where the power can be detected. However, at high optical irradiance levels, the polarization dither applied to one of the fiber beams prior to the TFB combiner interacts via XPM with the other beams in the delivery fiber from the TFB combiner, which causes the polarization of those beams to change. Thus, depending on the length of the delivery fiber and the optical irradiance, the XPM modulation causes increased cross-talk between the fiber beams affecting the polarization dither, which ultimately results in the inability to obtain an accurate detection of the polarization dither on a particular beam so that its polarization can be properly adjusted. This limits the ability of the fiber beams to be co-polarized at the input of the TFB combiner, which ultimately causes a significant portion of the light to be coupled into the delivery fibers from the TFB combiner that are not being used. With a typical delivery fiber mode field diameter of about 20 μm and meter class lengths, this effect becomes significant at about 1 kW power levels.
To further understand the XPM non-linearity discussed above, consider that the XPM generates a phase shift that is larger when beams are co-polarized than when they are perpendicularly polarized. Hence, each beam can induce a birefringent phase shift on the other co-propagating beams. Rotating the state of polarization (SOP) of one beam rotates the principle axes of the index ellipsoid and thus varies the birefringence, which can rotate the polarizations of the other co-propagating beams. One of the beams acts as a pump to effectively create a wave plate in the delivery fiber that can rotate the polarizations of the other co-propagating probe beams. If all of the beams are of a near-equal power, as would be generally expected in a CBC architecture, then each beam acts as a pump for all other beams. This cross-couples the SOPs and interferes with the polarization control, resulting in instability and a decreased coherent combination efficiency.
U.S. Pat. Nos. 8,922,771 and 8,922,772 issued to Goodno et al., assigned to the assignee of this application and herein incorporated by reference, disclose a multichannel optical polarization controller including a mixing device responsive to a sample beam and a reference beam that provides an in-phase signal including the mixed sample beam and reference beam with a 0° relative phase shift, and an in-quadrature signal including the mixed sample beam and reference beam with a 90° relative phase shift.
The '771 and '772 patents use phase dithers on each beam to identify a given beam at the combined output, and then provides feedback to correct the beam's polarization. In this implementation, the input polarization is dithered simultaneously on all input beams, but this causes concomitant changes in the SOP of the other beams through XPM coupling. Hence, at high power levels where non-linear effects in the delivery fiber are significant, the polarization dither information in the de-multiplexed signals is obscured by XPM-driven cross-talk from the polarization dithers of the other channels, rendering the control loop unstable. Using this polarization control technique, the XPM mechanism prevents effective beam combining and beam switching beyond 1 kW with meter-class lengths of 20 um core diameter delivery fiber after the combiner, thus constraining the output of all-fiber beam combiners in high power beam combined laser systems. Thus, there is a need for a fiber laser amplifier that delivers multi-channel coherent power using TFB combiners that does not suffer from XPM instability.